Principal applications of thermosetting unsaturated polyester resins include surface coatings and molding compositions which usually contain reinforcements, such as glass fibers. Such molding compositions are especially suited for the production of reinforced plastics. Among the many articles made from reinforced unsaturated polyester resins, the more spectacular are boat hulls and automobile bodies.
Since the unsaturated polyester resins are inherently brittle, attempts have been made in the past to overcome this disadvantage by incorporating plasticizers or flexibilizers, or other substances. Addition of components to polyester resins to improve toughness thereof has often proved to be unsuccessful due mostly to a significant deterioration of mechanical properties or incompatibility of such components with the polyester resins. It is, therefore, desirable to develop modified polyester resins which are toughened for crack and/or impact resistance by incorporating a compatible component which does not signicantly contribute to deterioration of thermal or mechanical properties of the polyester resins.